Radiant energy detectors are widely used in a variety of night vision systems for providing the capability of visually observing objects of interest which cannot be seen with the naked eye under conditions of very low light, as occur at night. Modern detectors of such radiant energy fall into three broad categories, namely, photoconductive, photovoltaic, and photo-emissive. These families of detectors are well known and are widely used. The two classes which are of primary interest in conjunction with the system of the present invention are radiant energy detectors of the photoemissive type, typified by photomultipliers and image intensifiers, and the photovoltaic type, exemplified by charge coupled detectors (CCDS) or charge induced detectors (CIDS). These particular classes of detectors have a spectral sensitivity which is compatible with infrared radiant energy.
Detectors of this type are extensively used in devices known as image intensifiers. Although many variations of image intensifiers exist, a class of such intensifiers which is in wide use and which illustrates the characteristics of such devices consists of an objective lens, a photocathode, and a phosphor screen followed by a viewing lens. An appropriate housing and a power source to activate the device are included. Image intensifier systems of this type function as follows:
The user of the device focuses the objective lens upon a certain object of interest. Typically, the object cannot be seen with the naked eye, because of the dim, nighttime light conditions. The focused radiation from the object is directed to a layer of material (the photocathode) which has the property of converting the focused incident radiation from the target object into photoelectrons. This section of the device generally is in a vacuum environment. The photoelectrons are released into the vacuum, and, as a result of the focused radiation, are caused to be focused and accelerated into a phosphor layer. This layer provides a radiation output in response to the photoelectron intrusion. Radiation amplification is provided through the energy added to the photoelectrons through the acceleration means; and when the phosphor screen is viewed through the viewing lens, the object is clearly seen.
Other more sophisticated devices for providing amplification in image intensifiers are presently available through the use of components such as microchannel plate amplifiers. These and other photoelectron amplifying devices are described extensively in current literature.
In summary, image intensifier devices serve to focus an electro-optical system onto a dimly lighted target object, and amplify the radiant energy from that object sufficiently that the object becomes more visible through the viewing lens of the image intensifier device. Such devices, however, have severe limitations. A dominant limitation is the inability of such devices to provide an amplified image of an object in circumstances where the radiant energy from the object is too low for the detector system to detect. For example, some night vision systems depend upon reasonable levels of starlight reflected from the object in order to present the viewer with a useful, recognizable image. In the presence of overcast, available starlight is suppressed or significantly reduced; and the image intensifier or amplifier function cannot respond. In daylight conditions, of course, such devices are not used since adequate acquisition of an object is available through ordinary means. Another environment in which image intensifiers of the type described do not work is at night in enclosed buildings such as warehouses, storage lockers, and the like, since there is no dim source of light which can be amplified by the image intensifier device.
A patent which is directed to a system for close range image enhancement is the Patent to DeBurgh #3,781,560. This patent discloses an infrared night vision device in the form of night vision goggles, coupled with an infrared light source for illuminating close objects, such as the pages of a book being read. There is no disclosure in this patent, however, of any medium or long distance image enhancement, nor is there any disclosure of the utilization of an infrared light as a point source target indicator.
A Patent to Bishop #4,027,159 discloses an image enhancement device using near infrared radiation from a source which functions in conjunction with a far infrared detector. The far infrared detector, however, is not a visual detector, but instead produces an audio output.
A Patent to Hadani #4,467,190 is directed to night vision binoculars which have a built-in image intensifier in them. These binoculars are typical of such night vision equipment. No supplemental infrared light source, however, is utilized or disclosed for use in conjunction with the binoculars. The binoculars simply intensify the infrared radiation from an object under observation. Infrared illumination of the object by some supplemental source is not contemplated in the device disclosed in this patent.
Solid state light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs) presently are well known and are utilized for a wide variety of applications. The LEDs generally provide output radiation over a relatively wide angle, whereas the LDs (laser diodes) provide a narrowly focused radiation. For infrared radiation at wave lengths which are compatible with most presently available night vision image enhancement devices, light emitting diodes and laser diodes based on Gallium Arsenide or Gallium Arsenide Aluminum materials are of primary interest. These devices are relatively small and operate on low amounts of power. They do not emit light which is visible to the naked eye, so that if they are to be observed, night vision detectors must be used to see them or to see objects which are illuminated by them. A patent disclosing the structure of such a Gallium Arsenide light emitting diode is the Patent to Ing #3,443,166. This patent discusses the utilization of the diode in conjunction with microelectronic circuits of the type used in computers and the like.
Optical zoom lenses of various constructions are commonly employed in telescope equipment used in conjunction with cameras, binoculars and the like. Such zoom lenses are employed to change the magnification power of the system and frequently are used in photographing or watching outdoor sports activities, wildlife, etc.
It is desirable to provide an infrared illumination system which can be used to supplement or be substituted for low light conditions to expand the usefulness of image enhancement devices. In addition, it is desirable to provide an infrared illuminator with zoom characteristics to expand or compress the field of illumination from an infrared laser diode light source.